JPS6138554A - Nuclear magnetic resonance projection device for permanent magnet - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Object of the Invention Industrial Field of Application The present invention relates to an apparatus for measuring nuclear magnetic resonance (NMRl), particularly an apparatus for imaging biological tissues, and more particularly, to relating to such a device substantially produced by.

BACKGROUND OF THE INVENTION In recent years, advances in nuclear magnetic resonance (NMR) technology have made it possible to image the two-dimensional and three-dimensional spin density of solids and liquids. Also, a number of new complex variants have been introduced into the rapidly expanding field of imaging. One important aspect of all these developments is the ability to image biological tissues in vivo. Does not affect healthy tissue and is normal
The radiation risk is much lower than the V31 imaging method.

In addition to producing images of spin density, all of these new NMR techniques can measure the spatial variation of spin/-lattice relaxation times in a specimen. For example, water in cells in cancerous tissue is known to have a longer spin-lattice relaxation time than in healthy tissue. Thus, NMR imaging, although in its infancy, holds promise as a diagnostic tool for early detection of tumors.

All of the NMR imaging techniques described rely on the preparation and/or observation of nuclear spinosystems in the presence of one or more magnetic field gradients. The field gradient serves to spatially differentiate areas of the specimen by the Larmor resonance frequency of spins from one area of the specimen to another.

Individual protons or hydrogen nuclei are found in most organic and biological materials and account for 99.9844%
It has a natural isotope abundance of . Other 0.015 of nuclear site
6% is accounted for by another naturally occurring deuterium isotope, seutilium.

Each nucleus has associated with it a small magnetic moment and a certain amount of angular moment called spin. Classically considered, the combined effect of the magnetic moment and the spi/ is the direction of the applied static magnetic field, just as a rotating top, when perturbed from its upright position, precesses in the direction of the gravitational field. Let the protons precess. For protons, the precession frequency is independent of the tilt angle of the magnetic moment with respect to the static magnetic field and is called the Larmor angular frequency ωO. However, it is the relation ωo =
r 13 o depends in fact directly on the magnitude of the static magnetic field f3o, where the constant is called the gyromagnetic ratio. This relationship is the key to much of what follows. When linear magnetic field gradients are otherwise spatially uniform (superimposed on A description of the development of NMR images and images can be found in Mansfield, Contemp.
, Phys, Vol. 17゜Circumstance 6. PP, 5
53-576 (1976). The basic principles of NMR necessary to understand the images are discussed and illustrated along with examples of images of the spin distribution of protons in a number of biological specimens.

i) NMR imaging for medical diagnostic purposes presents problems that raise the specter of anomalous properties for magnet designers. Hou, et al., Rev.

SCi, nstrLm', 52 (9), PP
, 1342-51Sept, (1981), preferably with a homogeneity of I PPm of at least 0.05 mm over important areas of the patient, such as the head or torso. A magnet is needed to generate the IT field.

Furthermore, as stated by HotLtt etαt,
A linear magnetic field gradient of 10-2T,,I in either direction may be required. Short-term field stability of better than 0.1 ppm may be enforced over periods of 1 s to avoid phase noise on the NMR signal, but long-term stability is about I PPm It requires that. Furthermore, all must be accomplished in a hospital setting. In hospitals, significant perturbations are caused by large amounts of rope in the building (reinforcement, water pipes, etc.) and by other ferromagnetic objects that may be moved (elevators, beds, nearby trucks, etc.). will occur. Iron core electromagnets have not been used because it is difficult to obtain the necessary magnetic field uniformly and the weight of these magnets becomes excessively large. Current designs are therefore air-core electromagnets, generally of resistive or superconducting design.

The spherical electromagnet used for NMR imaging is HoLLlt.

et, αl, , su, prα.

NMR superconducting magnet for in-vivo imaging is Gold
-srrth, gt, al,, Physiol
, CAgm, & Phys.

9PP, 105-107 (1977). In addition, Hanley is a member of the International Symposium on Nuclear Magnetic Resonance Imaging.
ympo-st, wm orLNucletLr M
agnetic Re5onαCeImaging,
Bowman Gyαy 5choo in 1981
lof Medicine, Wake-Fores
t University.

We discussed superconducting and resistive magnets in superconductivity in NMR scanning in a paper presented at the 2016 Winston-3Alem, N.C.

Superconducting magnets can generate much larger magnetic fields than simple electromagnets, but they are very expensive.

Permanent magnet systems would be superior to prior art superconducting and resistive electromagnets by: (α) requiring a means to generate the power required to maintain the field as in resistive magnet systems; I don't.

(b) There is no need to provide the necessary cooling means to remove heat as in resistive magnets or to maintain cryogenic temperatures as in superconducting magnets.

(The field of a C1 permanent magnet is not subject to power drift like that of a resistive magnet or a superconducting magnet not operated in resistive current, mode.

(d) does not decay gradually like that of superconducting magnets not operated in resistive mode;

(gl The material used is the frequency in question (5Mz~15
It can be a readily available ferrite magnet material that is transparent to electromagnetic waves (MHz).

The strength of the external field decreases rapidly with distance from the magnet, significantly reducing interference with bias fields from ferromagnetic objects in the vicinity of the device.

NMR permanent magnet designs that use ferromagnetic pole structures do not use the permanent magnet's magnetic flux potential efficiently. Thus, a novel permanent magnet NMR design that efficiently uses magnetic flux potential is desired.

SUMMARY OF THE INVENTION The present invention provides a permanent magnet NMR imaging system for imaging biological tissue.

Configuration of the Invention The permanent magnet NMR imaging apparatus according to the present invention includes bias means for generating a bias field, means for generating a gradient field,
and radio frequency means for applying pulses of electromagnetic radiation to nuclear spins associated with biological tissue at their Larmor frequency and detecting signals emitted from said tissue; wherein said biasing means comprises a plurality of bipolar child ring magnets, each dipole ring magnet consisting of a plurality of segments, each segment consisting of oriented anisotropic permanent magnet material. The NMR imager of the present invention does not use ferromagnetic pole structures and efficiently uses magnetic flux potential.

Preferably, each dipole ring magnet has a loose axial orientation (2), where θ is the segment's radial symmetry line and the X-axis (which lies in the plane of the dipole ring magnet). and α is the angle between said radial symmetry line and the X-axis. .

According to the present invention, an NMR imaging device is provided in which the bias magnetic field is generated by a permanent magnet dipole.

The bias magnetic field is substantially stationary and uniform.

As used herein, a "substantially stationary and uniform magnetic field" is a magnetic field that is sufficiently homogeneous and sufficiently stable to provide an image of the desired biological tissue. be.

Preferably, the bias magnetic field is 5 x 10-' in the image volume or space of the design in which the test specimen is placed.
More preferably the bias field variation is less than 1×10 −4 . Furthermore, the field stability is preferably no worse than 5 x 10-6 (5ec-1). However, as mentioned above, these millimeters can vary depending on the desired acceptable quality.

The invention will be further described with reference to the accompanying drawings. 1st
The figure shows an NMR imaging device 10 according to the present invention having an aperture of sufficient diameter to receive a person in question for scanning.
shows. NMR imaging apparatus 10 consists of four collars or rings 20, each ring consisting of a dipole magnet made of permanent magnetic material.

Each ring or collar 20 consists of eight segments 22 of permanent magnetic material. More or less segments can be used.

However, %8 segments provides very satisfactory results in the described embodiment. ``Permanent magnetic materials are oriented, anisotropic (αrLisot-γop
ic) permanent magnetic materials, such as rare earth/cobalt materials or ferrite ceramic materials. A preferred material has a transmittance of 1 (tLrLity perme −
αhility). Suitable materials include, for example, samarium cobalt, barium ferrite, strontium ferrite, and the like. Conveniently, each segment 22
has a trapezoid.

Each segment is constructed from individual blocks (brLck) 23 to the desired size, as shown in FIG. Each segment 22 may also be made from a solid block of such permanent magnetic material. block 2
3 are located along the surface of each segment 22 and cut appropriately to provide the desired final shape. Each segment 22
The individual blocks 23 are laid down in a mold 25 to form.

Type 2.5 is conveniently made from fiberglass. The surface of each block 23 that will come into contact with the surfaces of other blocks is coated with adhesive before laying the block in the mold 25.

An unmagnetized block with dimensions of approximately 15cm x 10crIL x 2.5cIIL is used for each segment 2
It is conveniently used to configure 2. Trim the blocks to ensure they have sharp edges that fit tightly together in the mold and leave no air gaps. The block is thoroughly cleaned leaving no oil, grease and loose material.

Two-component adhesives have proven convenient. The surface of one block, for example, Loctite Loquic Ply? -(Loctitg Loqu, cc Prt
mtr) coated with N or its equivalent. The surface of the second block in contact with the coated surface of the first block is then coated with, for example, Loctitt 5aperbonder 3.
25 or its equivalent. After the block is placed in the mold 25, the block is allowed to cure for a sufficient period of time so that the segments 22 can be removed from the mold 25. Typically about 10 minutes is sufficient for initial curing. However, this time will vary depending on the particular adhesive used and parameters such as temperature. Thoroughly clean the pre-cured segments with degreaser and allow sufficient time for the adhesive to fully cure.

Segment 22 is then placed within the magnetizing coil of the magnetizing device. The magnetizing device is capable of producing a field of one pulse throughout the volume of the segment at least 8 kiloel steps to completely magnetize the permanent magnet material. Positive segment.

With the new orientation, the formula α=20−(
Clamping the segments of the ring 20 according to Rule/2) as described above, depending on the predetermined positions of the segments of the ring 20.

Segment 7m5-clamp using suitable fittings made of non-ferromagnetic, non-conductive material. After pulsing the magnetizer to magnetize the permanent magnet material, the segment is removed and the aluminum support plate 27 is bonded to the segment 22 using epoxy resin 28 or the like, as shown in FIG. join to.

Each segment 22 is then clamped at its outer stop with a positioning ram 30, as shown in FIGS. 8 and 9. The thin adjustment screw 32 on each ram 30 is fixed near the center of its travel, as shown in FIG. When all eight segments are positioned in their rams 30, according to the predetermined alignment of the gradual axes of each segment, the rams are advanced to the innermost position as shown in FIG. do.

The initial design and optimization of an NMR imaging device according to the present invention is illustrated with respect to the configuration shown in FIG. 1st
Consider four ring dipoles arranged along the axis as shown in Figure 5. The field ByCx, y%W) is defined as: 'ty'-2 ((x2+y2)+2/
y2Cx2+y”)No1))HA Cqpp)=
Gg(p>-q Gg(p/q> where X%y and W are coordinates in space at any point, W axis is the axis of the ring dipole, N is the axis of the ring dipole is the number of
And 1 is the remaining field. The variables α, b and 2 are
Physical parameters of ring dipole configuration, as illustrated for four ring dipoles in FIG.

The magnetic design for the NMR imager is optimized according to the flow diagram shown in FIG. Uo
, U3 and h/α: Uo=0
．． 05.0.10.0.15.0.20; U3-0.9
;b/α=18.2.0,2.2.2.5° The expansion of the field about the axis near the center of the configuration has the form.

By=B, +B2 (z/α)2+B4A(z/a)'
+B6(z/α)6+... Adjust the magnetism/remeter so that B2=B4=o. B2 and B4 are approximately 0. When, B.

and calculate B6. Desired B by changing the values of Uo, U3 and b/α. and the allowed distortion C field non-uniformity), we get the product, which is equal to B6.

The existence of radial distortions within acceptable limits is checked by analyzing the field distortions at points away from the solid axis. Small amounts of B and B4 so as to reduce the distortion in the extremes of the working volume at the expense of a small increase in distortion in the internal areas of the working volume, thus reducing the overall distortion in the working volume.
Introduce a term.

The procedure described here is typically used to obtain optimal field uniformity in the image field, but if conditions do not require optimal field uniformity less than optimal. It can be used to get sex money.

After the design has been optimized by the above procedure, the individual ring dipoles are assembled using the fixtures shown in FIGS. 8 and 9 and tuned by the following procedure.

The radial position of the ring dipole segment 22 is determined by measuring the magnetic flux density within the image volume using a Hall effect probe 60, as shown in FIG. Adjustments are made to eliminate inhomogeneities in the dipole magnetic field within the designed imaging volume 65.

First, the harmonic content of the field for each of the first eight harmonics (corresponding to eight segments) of the magnetic field in the designed image field term
contourLt) f, determined by measuring the magnetic flux density at a series of points. The first segment is then moved radially a small distance by the fine adjustment screw 32 of the positioning ram 30. The harmonic content of the magnetic field within the designed image volume 65 is remeasured. The segment 22 is then returned to its initial position. Each cementonote is sequentially displaced radially by a small distance and the harmonic content of the magnetic field within the designed imaging volume 65 is measured. all 8
After displacing one segment and measuring the harmonic content, an 8×8 sensitivity matrix
ix). This sensitivity matrix indicates the sensitivity of the harmonic content of the magnetic field within the imaging volume of the design for each segment of the ring dipole.

The elements of the matrix are defined by the following equation: Oγ1 where δA1. rL is the change in the harmonic content of the 1st harmonic due to a change in the position of segment 1, and δr1 is the change in the position of segment 1'.

After the sensitivity matrix is calculated, the inverse of the sensitivity matrix, i.e.
Calculate the correction matrix. Remeasure the harmonic content of the field within the image volume of the design. The harmonic content for each harmonic is then calculated as the theoretical harmonic content for each harmonic (i.e.,
The difference vector (d, 1
vector). Next, the difference vector is multiplied by a correction matrix to perform tuning correction (turLing).
correctorL), i.e., the image volume of the design contributed by the tuned ring dipole, to the ideal, i.e., harmonic content of the design of 65, each segment 22
to obtain the distance and direction that must be moved to approach more closely. Each segment/toe is then moved by the calculated amount and the method is repeated so that the harmonic content is within specifications, ie, the magnetic field homogeneity is within design specifications.

The collar structure (not shown) is then mechanically fastened to the support plate of each segment by fastening the collar to the support plate of each segment.
Attach to the ring dipole segments by screws or adhesive. The dipole ring with the segments fixed by the collar assembly is then removed from the positioning ram and attached to the rib guide 41 as shown in FIG.
assembly-fitting 40 by lowering it along.

Each successive ring dipole is tuned as described above and the number of ring dipoles in the design, in this case four, is set at the assembly/pulley fixture 40 as shown in FIG. into the assembly fixture 4o until it is placed inside the assembly fixture 4o. At this point, the guide arm actuation mechanism 4
As each ring dipole is drawn into position adjacent to the previous ring dipole by 5, a shim 42 will be placed between each of the ring dipoles. Because of the repulsive forces between each ring dipole, each ring dipole must be mechanically locked in place before the next ring dipole is placed into the assembly/pulley fixture 40. After the four ring dipoles are in place, safety stop 46 is bolted in place.

The device 10 must now be axially tuned.

The field strength along the axis of the device 10 can be expressed as a power series of the following equation: B2 = 2 = 2c 2 n where Bz is the field at point 2 on the axis, and C1
is the axis coefficient. The coefficients for the first three axes are determined by measuring the magnetic flux density at various locations along the axes using a Hall effect probe 60. The thickness of the tuning shim 42 is then varied. 1st dipole and 2nd dipole
By varying the separation with the dipole, the position of the first ring dipole is varied with respect to the designed imaging volume.

The coefficients of the axes are then redetermined. The first synchronized sim f! : Swap between the first ring dipole and the second linog dipole, and change the distance between the second ring dipole and the first ring dipole. After determining the axis coefficient of change, the first limb is exchanged and the method is repeated for the separation between the third ring dipole and the fourth ring dipole. Calculate the sensitivity matrix similarly to radial tuning,
Here the elements of the matrix are the changes in the axis coefficients divided by the changes in separation.

Calculate the inverse of the sensitivity matrix, i.e., the correction matrix, and the difference calculated from the measured axis coefficients and the design axis coefficients (
Hang the kutle. The above multiplication can be calculated to correct the separation between the ring dipoles.

until the magnetic field with the design image volume falls within the design specifications.
Repeat this method.

When the magnetic field within the design imaging volume falls within the design specifications for homogeneity, permanent shims are machined from a suitable non-ferromagnetic material to maintain the desired separation between the ring dipoles. After the permanent shims are placed between the ring dipoles, the structural beam 4
The ring dipole forces 2- are mechanically fixed together by bolting or welding 7 to the collar of the ring dipole.

removing the device 10 from the axial assembly fitting;
It is placed on a substrate 50 as shown in FIG. Permanent magnet interest reversibly changes magnetism as temperature changes.

If the temperature changes by more than approximately 1° anywhere within the magnet,
The field will exhibit temperature distortion. This can be prevented by placing 3α insulation (eg, urea-formaldehyde resin) over the entire exterior surface of the finished magnet, as shown in FIG.

Attach the end cover 54 and side panels 55 to the finished look basin.

NMB imagers with 3.4 and 5 ring dipole configurations as shown in Figures 4A, 4B, 4C and 4D were optimized in a similar manner. and can be synchronized.

The NMB imaging system according to the invention also has a gradient field superimposed on the bias field provided by the Linda dipole device described above. The gradient field can be provided by any means for providing a gradient field for conventional NMR imaging systems where the bias field is provided by an electromagnet.

[An air core (αtr core) current gradient coil system is preferred.
tgrbur et al, 5tate Univer.
City of New York, 5tony Br
It can be located outside the bias magnet, as in the NMR imaging system of ook), or it can be located inside the bias magnet adjacent to the imaging volume.

Power to the gradient coils is provided as shown in the block diagram of FIG. A fixed DC system power supply (SPS) 70 is connected to an output amplifier 71
is connected to the coil system 72 by f.

The actual field gradient is controlled by the microprocessor 75. The microprocessor determines the polarity of the pulse, ξ
It is programmed to control pulse height, pulse width, pulse shape, and usage cycle.

Radio frequency (RoF, ) coils detect the nuclear magnetic moments of hydrogen atoms in biological tissues.

The RIF, system is based on well-known techniques, e.g. Hoult's paper (Hoult, “Rad, io Freq
u, encyCoil TtchrLology in
NMRScarLning”, 1981 Inte
rnational Sympostum onM
u, clear Magnetic Re5onanc
e Submitted in Imagtng, Botnman G
ray 5chool of Medtcine.

Wake-Forest Universe, ty,
WirLston-8atgm, N.C.).

FIG. 18 is a block diagram of the R0F' system. It is divided into eight sub-systems. Secondary system 1, programmer function h, microprocessor computer, e.g. LSI-11
Its job is to take commands from the microprocessor controller and translate them into appropriate voltage signals needed to operate the various gates, phase shifters, etc. in the system. Secondary system 2, the transmitter, under the control of the programmer, sets the appropriate frequency, phase and
pe) to excite the spin/stem. Secondary system 3, a power amplifier, amplifies the RoF pulses provided by the transmitter and matches their inopedance to the transmitting antenna.

The secondary system 4 includes a transmissor and register bar and a secondary T/R switch (whether they are of the same or separate construction) for transmitting and receiving antennas. Cali plater,
The secondary system 5 checks the sensitivity of L/'/bar and the gain drift (,!7α1rLdrift)
Provide a signal of correct frequency and known strength to be injected periodically into the receiver to prevent problems. Secondary system 6 is a receiver that senses the NMR signal and converts it into a usable signal. Finally, the secondary system 7 converts the analog signal from the receiver at the output interface into a digital signal that can be fed to the imaging computer.

The imager/computer reconstructs two-dimensional or three-dimensional images from data obtained as a function of varying magnetic field gradients according to known techniques, such as a Fourier transform.

Although the present invention has been described in detail with respect to an NMR imager having eight segments of permanent magnetic material, each consisting of four ring dipoles, each forming a substantially continuous ring, the method described above has been described in detail. It is equally applicable to such systems with more or fewer ring dipoles, and to ring dipoles made from more or fewer segments. In fact, the most preferred axial arrangement of the ring dipoles is one positive ring dipole in the center surrounded by two small negative ring dipoles and then a large ring dipole, as shown in Figure 4D. Consists of a ring dipole.

However, for practical reasons, the arrangement shown in FIG. 4B is preferred for practical devices.

[Brief explanation of the drawing]

FIG. 1 is an isometric view of a permanent magnet NMR imaging apparatus according to the present invention. FIG. 2 is a partially cutaway front view of the NMR imaging apparatus shown in FIG. , FIG. 3 is a cross-sectional view of the apparatus of FIG. 2 taken along line 3--3 of FIG. FIG. 4A shows an NM of the present invention with three ring dipoles.
1 is a sketch showing one embodiment of an R imaging device. FIG. 4B shows an NM of the present invention having all four ring dipoles.
1 is a sketch showing one embodiment of an R imaging device. FIG. 4C is a sketch showing one embodiment of an NMR imager of the present invention having all three ring dipoles, with the center ring dipole having a smaller outer radius than the outer ring dipoles. FIG. 4D is a sketch showing one embodiment of an NMR imager of the present invention having 5° ring dipoles, two of which are made of a material with a negative residual inductor female, Br. . FIG. 5 is a front view of a segment of permanent magnetic material formed from individual blocks. FIG. 6 is a cross-sectional front view of a mold in which individual blocks are laid to form segments of permanent magnetic material of the desired size and shape as shown in FIG. FIG. 7 is an exploded isometric view of a segment of permanent magnetic material with a support plate. FIG. 8 is an isometric view of the segment shown in FIG. 7 mounted on a positioning ram to form a dipole ring magnet. 9 is a side view, partially in section, of the positioning ram of FIG. 8; FIG. FIG. 10 is a top view of a plurality of positioning rams mounted in a jig for forming a dipole ring magnet. FIG. 11 is an isometric view of a dipole ring magnet arranged with respect to the imaging volume, illustrating the constant of magnetic flux density within the imaging volume contributed by the ring dipole. FIG. 12 is a partially sectional front view of a fixture for adjusting the spacing between dipole ring magnets for tuning an NMR imager, with one dipole ring magnet already in place. 13 is a partially sectional front view of the fixture of FIG. 12 with four dipole ring magnets mounted therein and a safety stop in place; FIG. FIG. 14 is an isometric view of an assembled NMR imaging device according to the present invention. FIG. 15 is a cross-sectional sketch of a four ring dipole showing the physical parameters. FIG. 16 is a flow diagram of a method for optimizing the design of a permanent magnet ring dipole system for NMR imaging. FIG. 17 is a block diagram of a gradient coil for NMR imaging. FIG. 18 is a block diagram of the R,F' system for NMR imaging. 22... Segment 40... Assembly fixture 23... Block 42... Shim 25...
Mold 46... Safety stop 27... Support plate 47... Structural beam 28... Epoxy resin 50... Substrate 30... Positioning ram 54.
・・Cover -+-3 FIG, 3 FIG, +2 FIG, +5 FIG, 17

Claims (1)

[Scope of Claims] 1. having a desired imaging volume for imaging biological tissue, bias means for generating a bias field, means for generating a gradient field, and pulses of electromagnetic radiation for directing pulses of electromagnetic radiation to the biological tissue; and radio frequency means for detecting the resulting signal emanating from said tissue, wherein said biasing means comprises a plurality of dipole ring magnets, each dipole ring magnet consisting of a plurality of segments, each segment comprising: , anisotropic oriented permanent magnetic material arranged in rings such that there is a substantially continuous ring of permanent magnetic material. 2. The bias field is generated by 2, 3, 4 or 5 dipole ring magnets.
MR imaging device. 3. The NMR imaging apparatus of claim 2, wherein the two external dipole ring magnets have a larger outer radius than the remaining dipole ring magnets. 4. The NMR imaging apparatus of claim 1, wherein each dipole ring magnet comprises eight segments of permanent magnet material. 5. The NMR imaging apparatus according to claim 4, wherein each segment is substantially trapezoidal. 6. The NMR imaging apparatus according to claim 1, wherein each segment is composed of a plurality of small blocks. 7. The bias field within the desired image volume is 5×10^-^
2. An NMR imaging system as claimed in claim 1, having a field uniformity, ΔB/B, of less than 4. 8. A patent in which the dipole ring magnets are positioned axially with respect to each other to obtain a predetermined field inhomogeneity over a predetermined central imaging volume with a minimum mass of permanent magnet material. An NMR imaging apparatus according to claim 1. 9. The dipole ring magnets are positioned axially with respect to each other to obtain a minimum field inhomogeneity over a predetermined central imaging volume with a predetermined mass of permanent magnet material. An NMR imaging apparatus according to claim 1.